1. Field of the Invention
The present invention relates to an objective lens system for a microscope of a liquid immersion type.
2. Related Background Art
Hitherto, there has been known a microscope objective lens in an oil immersion system, the arrangement being such that an oil fills in an optical path between a sample and the microscope objective lens in order to enhance a resolution. This type of known oil immersion system microscope objective lens is disclosed in, e.g., Japanese Patent Application Laid-Open Nos. 59-155822 and 61-240218.
When observing the sample by use of the above-described oil immersion system microscope objective lens, it is required that a space between the sample and the microscope objective lens be filled with the oil. For this reason, there is a possibility in which the oil is adhered to the microscope body and the objective lens in the oil immersion system microscope objective lens, resulting in a troublesome maintenance after the observation. Further, there exists such a possibility that a hand of the observer may be contaminated with this oil. The thus adhered oil is hard to clean off easily, and this may cause a problem to deteriorate the operability during the observation.
Then, if the water is used instead of employing the oil, the above deterioration in terms of the operability is to be obviated. The water, however, unlike the oil for the oil immersion, has a large difference in refractive index with respect to a cover glass covered on the surface of the sample. Accordingly, a spherical aberration is produced from this cover glass, with the result that a contrast of a sample image to be observed largely declines. Under this condition, it can be thought to optically design the microscope objective lens in consideration of the spherical aberration produced from the cover glass. If the thickness of the cover glass or the refractive index changes due to an error in terms of manufacturing, however, the spherical aberration caused by the cover glass also changes, and hence there arises a problem in which the well-formed image can not be obtained at all times.